In the future, it would be advisable to tell your host that you cannot hold open her party due to the possibility of this happening, but for now, it is something that has to be dealt with. This becomes more common as we get closer to holidays, but it can happen anytime, even overnight.
The first thing you have to do is call your customers and explain the situation. If there is an alternative that you feel would work well, start by suggesting that they may prefer to get product B instead of product A. If product B costs more, you may want to eat the cost depending on how much it is. If product A costs more, then you would refund the customer the difference.
If there is no alternative, explain this to them and ask if there is something they would prefer to buy. Sometimes they will come back with, “oh yes, I wanted to buy Item Z but I only had so much money”. Then you can put this on her order and refund or charge her more as need be.
Sometimes the customer just does not want anything else but a refund. If they paid by credit card, a refund is easy, you just charge their card less if you have not processed it. Cash or cheque would have to be refunded when the product is delivered. With a cheque, you need to cash it to get the money you need, unless they have cancelled their entire order.